请实现一个js脚本，要求做到将数字转化为千分位表示如：1234567转化为1,234,567

//法一
function parseNum(num){
    var list = new String(num).split('').reverse();
    for(var i = 0; i < list.length; i++){
        if(i % 4 == 3){
            list.splice(i, 0, ',');
        }
    }
    return list.reverse().join('');
}

console.log(parseNum(10000121213));


//法二
function parseNum(num){
    var reg=/(?=(?!)(d{3})+$)/g;
    return String(num).replace(reg, ',');
}
console.log(parseNum(10000121213));


//法三
String.prototype.strReverse = function(){
    return this.split('').reverse().join('');
}

function parseNum(num){
    var str_num = String(num);
    var len = str_num.length;
    var tail = str_num.slice(0, len % 3);
    tail = tail.strReverse();
    var reg=/d{3}/g;
    var list = str_num.strReverse().match(reg);
    list.push(tail);
    var res = list.join(',').strReverse();
    return res;
}
console.log(parseNum(10000121213));


//法四
function parseNum(num){
    var list = String(num).split('').reverse();
    var temp = [];
    for(var i = 0, len = list.length; i < len; i = i + 3){
        temp.push(list.slice(i, i + 3).join(''));
    }
    return temp.join(',').split('').reverse().join('');
}
console.log(parseNum(10000121213));